In general, ceramic fine particles include fine particles of oxides, such as TiO.sub.2, ZnO, Al.sub.2 O.sub.3, and SiO.sub.2, those of carbides, such as-SiC and TiC, and those of nitrides, such as Si.sub.3 N.sub.4, TiN, and AlN. Having various properties, such as photoconductivity, piezoelectricity, fluorescence, and catalytic effects, the fine particles are frequently used in various industrial fields.
Among the ceramic fine particles, titanium oxide fine particles, for instance, have diversified industrial applications, and they are used as white pigments, magnetic starting materials, abrasives, pharmaceuticals, and ultraviolet shielding materials.
Also, SiC fine particles, for instance, have diversified industrial applications, and they are used, for instance, for thermistors utilizing their resistivity stability and for varistors utilizing their non-linearity of voltage-current relationship. Other ceramic fine particles also have various industrial applications.
As mentioned above, although ceramic fine particles have very great industrial applications, it is important to produce uniform fine particles with a narrow particle diameter distribution in order to optimize their performance. In other words, by producing fine particles, specific surface areas thereof are increased, and the proportion of number of molecules located on the surface of the fine particles based on the entire number of molecules constituting the fine particles increases. Therefore, the surface energy of the fine particles is increased, and the performance of the fine particles per unit weight is markedly enhanced. In addition, no variation in the performance of the fine particles is found, because the particle diameter distribution is narrow.
Also, by forming a concentric, multi-layered structure, the ceramic fine particles can enjoy the combined functions of the ceramic constituting the respective layers, and the improved properties for the surface of the substances forming the core. In order to exhibit an optimum performance of the fine particles having a multi-layered structure mentioned above, each of the layers constituting the ceramic fine particles has to be uniformly coated. By uniformly and evenly coating the outermost layer, in particular, it is possible to inhibit the properties of the surface of the inner layer from directly affecting the outer portion, and to uniformly exhibit the properties of the outermost layer.
The methods for producing ceramic fine particles of having extremely important industrial applications as mentioned above can be roughly divided into a liquid-phase method and a vapor phase method.
Examples of the liquid-phase methods include a method for producing zinc oxide fine particles comprising hydrolyzing a metal alkoxide thereof, to give zinc oxide fine particles (Japanese Patent Laid-Open No. 2-59425). Also, in general, a long-time method used comprises adding an acid or alkali solution to a metal salt to cause a reaction in the liquid-phase, to give desired ceramic fine particles.
As for fine particles having a multi-layered structure, for example, a method comprising the steps of adding a metal salt to an aqueous suspension of, for instance, TiO.sub.2 fine particles, and coating the surface of the fine particles with a metal oxide by a neutralization reaction is known (Japanese Patent Laid-Open No. 3-88877).
The production processes based on the liquid-phase method are difficult to automate, because they are basically subject to a batch process. In addition, because the formed fine particles are obtained in a solid-liquid mixed phase, filtration and drying steps have to be added to give a finished product. Therefore, the entire production process becomes more complicated, and maintenance of the entire process becomes difficult, which in turn makes it difficult to lower product cost.
The vapor-phase methods include a method for producing ceramic fine particles generally comprising the steps of vaporizing a metal, and mixing the formed vapor and an oxygen-containing gas to carry out a catalytic oxidation reaction (For instance, methods for producing zinc oxide fine particles are disclosed in Japanese Patent Laid-Open Nos. 1-286919 and 2-208369). Also, there is a method for producing fine particles known as CVD method (chemical vapor deposition method), as one means for the vapor-phase method, the method comprising the steps of introducing gaseous starting materials together with a carrier gas into a reaction tube, and supplying energy by heating or other means to the gaseous starting materials in the reaction tube to cause a chemical reaction, to give fine particles. The fine particles obtained by this method have a high product purity and a relatively even particle diameter. Examples of this kind of methods include a method for producing an amorphous, spherical silica powder according to the CVD method using an organic silicon compound as a starting material (Japanese Patent Laid-Open No. 4-97907), and a method for producing coated fine particles comprising forming a fluidized bed comprising fine particles using a low-high frequency synthetic sound wave and coating the fluidized bed by the CVD method (Japanese Patent Laid-Open No. 64-80437).
As for a method having the properties of both the liquid-phase method and vapor-phase method mentioned above, there is a spray pyrolysis method, comprising the steps of atomizing an aqueous solution or organic solvent solution, each solution containing a metal salt of an inorganic acid or organic acid, conveying the atomized liquid particles to a heating furnace, and carrying out a pyrolytic reaction with the liquid particles, to give oxide fine particles (For instance, a method for producing an oxide superconductor is disclosed in Japanese Patent Laid-Open No. 2-196023).
In the above-mentioned vapor-phase method, or the CVD method, which is one means thereof, and the spray a pyrolysis method, which is a modified method thereof, in general, for the purposes of controlling the particle diameter and crystallinity of the produced fine particles in a reactor for producing fine particles, the fine particles are produced by supplying gaseous starting materials in a laminar flow into a tube type reactor to achieve an even temperature distribution and an even concentration distribution of the gaseous starting materials, or an even density distribution of the starting material droplets in the reactor. Therefore, it is difficult to obtain an even temperature distribution, concentration distribution of gaseous starting materials or density distribution of starting material droplets in the reactor when "scale-up" of the reactor takes place, so that scale-up of the reactor is made difficult. In addition, the yield is low because large amounts of fine particles are adhered to the inner wall of the reactor.
On the other hand, in the case of employing liquid-liquid reaction, a method for producing uniform droplets using Taylor vortex flow is disclosed in Japanese Patent Laid-Open No. 56-139122. Here, the Taylor vortex flow is a donut-shaped vortex flow exhibited in a fluid when the gap between double cylinders sharing the same center of axis is filled with the fluid and the inner cylinder is rotated at a given rotational speed or higher, while keeping the outer cylinder stationary. The Taylor vortex flow is regularly stacked in the axial direction of the cylinder, and it can be parallel-shifted in stacks without extinction by setting an appropriate axial flow velocity in the liquid phase. By utilizing the above-mentioned Taylor vortex flow, the scale-up of the reactor becomes easy, so that continuous reaction can be carried out. However, examples applying to either of the vapor-phase method or the spray pyrolysis method have not so far been known.
Although applications of the Taylor vortex flow to production of uniform droplets in a liquid-liquid system and to liquid-liquid reactions is disclosed in Japanese Patent Laid-Open No. 56-139122 mentioned above, a method for producing ceramic fine particles utilizing the Taylor vortex flow has not yet been known.